Accommodation enables a person of normal vision to focus on objects from infinity to a near point, typically of the order of 25 cm or less from the eye. Presbyopes lose the ability to accommodate over such a large range and typically require two or more corrections: a distance correction for focusing to infinity, a near correction for focusing to close objects, and occasionally one or more intermediate focusing corrections. Patients whose natural, crystalline lens has been excised (due to cataracts, for example) lose the ability to accommodate altogether.
Multifocal spectacles provide two or more corrections for each eye in separate regions of one lens. Such spectacles rely on the fact that the spectacle lenses are relatively fixed with respect to the eye, so that the spectacle wearer usually looks through the higher portions of the lens for distant objects, and the lower portion of the lens for near objects. Multifocal contact lenses and intraocular lenses that provide both distant and near corrections for a particular eye in a single lens are also known in the art. Unlike spectacle lenses, however, contact lenses and intraocular lenses move with the motion of the eye. A number of different approaches have been proposed to overcome this difficulty.
For example, U.S. Pat. No. 5,073,021, whose disclosure is incorporated herein by reference, describes a bifocal ophthalmic lens constructed from birefringent material. The dual focal property arises due to the differing indices of refraction of the birefringent material for light polarized parallel to the fast and slow axes of the material. Light emanating from far objects having one polarization and light emanating from near objects having the opposite polarization are both focused onto the user's retina. Thus, an in-focus and a blurred image may appear simultaneously on the user's retina. The ability of the user's eye/brain system to distinguish between the two images provides bifocal action from a single lens.
As another example, U.S. Pat. No. 4,300,818, whose disclosure is incorporated herein by reference, describes a multifocal ophthalmic lens, which provides a variable focusing power lens for near and distant corrected vision. The lens includes electrodes disposed between first and second lens elements, with a film of liquid crystal between the electrodes. A voltage is applied to the electrodes to vary the index of refraction presented by the liquid crystal film, and thus to provide a variation in the refraction and focal length of the lens elements.
Similarly, U.S. Pat. No. 5,712,721, whose disclosure is incorporated herein by reference, describes a switchable lens, whose focal length may be changed by application of an electrical or magnetic field. A switching means provides a drive impulse to change the focal length of the lens. An integral power source, such as a miniature battery or a photocell, provides power for the switching means. This arrangement permits a user of the lens to switch between near and distance vision.